US scientists have engineered a plant which glows when touched, creating a unique window into how plants sense being handled.

Plant geneticist Janet Braam, and her team at Rice University in Texas, discovered that plants have genes that respond to touch 10 years ago.

The bushy plant has been touched regularly

She showed how touching plants could even affect their physiology.

The low bushy one on the left of the picture (right) had been touched regularly, whilst the one on the right was not touched and grew much taller and more spindly.

"It was rather serendipitous," recalls Professor Braam. "We were looking at the effect on these genes which were turning on when we applied a hormone. Then we realised that it wasn't the hormone, it was the application of the hormone.

"It was the act of touching the plant that was turning the genes on."

Ultra-sensitive camera

It was well known that plants like the Venus Fly Trap responded to touch in dramatic ways. But, says Professor Braam, now we know that probably all plants react to touch, just less dramatically.

For the first time we can see in real time how the plant responds to touch

Prof Janet Braam

So far she has found five genes in common mustard plants which respond to being touched. In the early stages of her research they had to sacrifice the plants to see if the genes had switched on.

The approach they are now taking is to add a firefly glow gene onto the touch genes. Now when the plants are touched, they glow.

"For the first time we can see in real time how the plant responds to touch," she says.

The mustard plants only put out a tiny amount of light, typically a thousandth of a billionth of a watt. So they need to be placed in the dark under an ultra-sensitive camera, to pick up the glow.

Chemical pathways

It takes about 20 minutes for these genes to start glowing strongly where the plant was touched. Then there seems to be a kind of chemical nervous system which spreads the information to other parts of the plant, switching the genes on there too.

The Venus Fly Trap responds to touch

The effect dies off after about an hour.

"It makes sense that plants should respond to touch," says Professor Braam. "Plants can't move, so they have evolved ways of acclimatising to a particular environment. If a plant's growing in a windy place it has more chance of surviving if it's short and stocky, rather than long and spindly".

In the future, the team are going to harness a different glowing gene from the jellyfish. This will allow them to look in more detail at the exact chemical pathways which spread round the plant from the moment it is touched.

The glowing plant is featured on BBC One's Tomorrow's World television programme which is shown at 1930 BST on Wednesdays.